The present invention relates to a light dosing device comprising a photosensitive element which is subjected to the light flux emitted by a source of light and which either controls the open-time of a diaphragm disposed in the beam path between the source of light and the element or the on-time of the source of light, according to the intensity of the light impinging upon a material to be exposed. The device further comprises an amplifier for amplifying the output voltage of the element and a comparing unit for comparing the amplified output voltage to a reference voltage.
When exposing photosensitive materials, e.g., printing plates, the exposure time is generally chosen in accordance with the characteristics of the material to be exposed by integrating the voltage directly or indirectly supplied by a photosensitive element. For this purpose, a saw-tooth is generated the slope of which changes as a function of voltage. For extended exposure times, a number of saw teeth are obtained. It may be empirically determined, for example, how many of these saw teeth are required for proper exposure of a particular type of material. This number is then preset in a light dosing device. Depending upon the light intensity, the photosensitive element supplies a more or less high voltage which, when integrated, results in saw teeth of different lengths and thus in exposure times of varying lengths with an unchanging number of teeth.
The principle of integrating voltage values is known from German Offenlegungsschrift No. 2,046,887 which refers to a photographic copying machine in which changes in the light intensity of the source of light are accounted for in the exposure time. This known equipment is provided with an electrical timer having an adjustable delay controlled by a photoelectric transducer in accordance with the light intensity measured at the original to be copied. The timer comprises a potentiometer for a direct voltage corresponding to the preset delay. The direct voltage is time-integrated by an amplifier and is compared to a reference voltage by means of a comparing unit which emits a control signal as soon as a particular voltage difference is attained. In this equipment, the light flux from the source of light is integrated and its changes are accounted for in the exposure time; however, the equipment is not adapted to use different reference voltages for materials of different sensitivities and to obtain a control signal for actuating the source of light or a diaphragm by comparing the integrated direct voltage of the source of light to a reference voltage without forming a voltage difference.